Method and composition for sealing a subsurface formation

ABSTRACT

A method for plugging and sealing subsurface formations using alkaline nanosilica dispersion and a delayed activation chemistry is disclosed. In accordance with one embodiment of the present disclosure, the method includes introducing a mixture with a first pH into the subsurface formation. The mixture comprises an aqueous solution, an alkaline nanosilica dispersion and a water-soluble hydrolyzable compound. The method further includes allowing the water-soluble hydrolyzable compound to hydrolyze in the subsurface formation to form an acid at 70° C. or greater, thereby acidizing the mixture to a reduced second pH and causing the alkaline nanosilica dispersion to gel into a solid and seal the subsurface formation. A composition for sealing a subsurface formation is also disclosed. The composition includes an aqueous mixture including water, an alkaline nanosilica dispersion, and a water-soluble hydrolyzable compound.

TECHNICAL FIELD

The present disclosure relates to natural resource well drilling andhydrocarbon production and, more specifically, to methods andcompositions for plugging and sealing subsurface formations.

BACKGROUND

Excessive water production greatly affects the economic life ofproducing wells. High water cut, such as a water cut of greater than50%, largely affects the economic life of producing wells and is alsoresponsible for many oilfield-related damage mechanisms, such as scaledeposition, fines migration, asphaltene precipitation, and corrosion. Insuch water production instances, methods and compositions for pluggingand sealing subsurface zones that lead to high water cut in wells can beutilized to decrease the water cut.

SUMMARY

It is often desirable to seal a subsurface formation to decrease waterproduction from that subsurface formation. Sealing the water-producingformation prevents contaminating hydrocarbon production from a producingformation with water from a water-producing formation. Thewater-producing formation may be either above or below the oil-producingformation. A high water cut leads to increased operating costs toseparate, treat, and dispose of the produced water according toenvironmental regulations. Though there are a variety of chemicals usedby the industry to control water production, most of them are notaccepted in the regions with strict environmental regulations.

The present disclosure provides compositions for plugging and sealingwater producing subsurface formations. Conventional sealing methodsutilizing silica to gel and seal the formation have pumped the silicaand acid compound separately. Pumping these compositions separatelyincreases surface equipment utilization and overall treatment time,which increases the cost of the operation. Furthermore, when pumpingacid in separately to gel the silica composition in carbonateformations, acid may be spent acidizing the carbonate formation, leavingless acid available to gel the silica composition.

Accordingly, there is a continuing need for a sealing method that willnot acidize carbonate formations and will result in low operating costscompared to conventional sealing methods. This need is met by thesealant mixture in the present disclosure, which includes both analkaline nanosilica dispersion and a water-soluble hydrolyzable compoundso the compositions are pumped into the formation simultaneously. Thewater-soluble hydrolyzable compound used to gel the nanosilica enablesthe sealant to be precisely placed into the target subsurface formationbecause the delayed activation chemistry gels the alkaline nanosilicadispersion into a solid once activated by the elevated temperatureinside the subsurface formation, of at least 50° C. The water-solublehydrolyzable compound forms an acid at temperatures of at least 50° C.In this way, the sealant mixture of the present disclosure successfullyavoids premature gelling within the wellbore or before the mixtureenters the formation, thereby avoiding premature plugging of pipelines,coiled tubing or other tubing. Furthermore, by including a water-solublehydrolyzable compound in the mixture, at least 90%, or at least 95%, orat least 99% of the acid will be used to gel the alkaline nanosilicadispersion, even in carbonate formations, and no more than 10%, or 5%,or 1% will be spent acidizing the carbonate formation. Lastly, silica isconsidered environmentally benign, and therefore may be used as asubsurface formation sealant in regions with strict environmentalregulations.

According to the subject matter of the present disclosure, a method forplugging and sealing subsurface formations using alkaline nanosilicadispersion and a delayed activation chemistry is disclosed. Inaccordance with one embodiment of the present disclosure, the methodincludes introducing a mixture with a first pH into the subsurfaceformation. The mixture comprises an aqueous solution, an alkalinenanosilica dispersion and a water-soluble hydrolyzable compound. Themethod further includes allowing the water-soluble hydrolyzable compoundto hydrolyze in the subsurface formation to form an acid at 70° C. orgreater, thereby acidizing the mixture to a reduced second pH andcausing the alkaline nanosilica dispersion to gel into a solid and sealthe subsurface formation.

In accordance with another embodiment of the present disclosure, acomposition for sealing a subsurface formation is disclosed. Thecomposition includes an aqueous mixture including water, an alkalinenanosilica dispersion, and a water-soluble hydrolyzable compound.

Additional features and advantages of the described embodiments will beset forth in the detailed description which follows. The additionalfeatures and advantages of the described embodiments will be, in part,readily apparent to those skilled in the art from that description orrecognized by practicing the described embodiments, including thedetailed description which follows as well as the drawings and theclaims.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The following detailed description of specific embodiments of thepresent disclosure can be best understood when read in conjunction withthe following drawings, where like structure is indicated with likereference numerals and in which:

FIG. 1 schematically depicts a creation of a seal in a subsurfaceformation according to one or more embodiments described in thisdisclosure.

DETAILED DESCRIPTION

As used throughout this disclosure, the term “anisotropy” refers tovariation in physical properties observed in different directions.

As used throughout this disclosure, the term “casing” refers tolarge-diameter pipe cemented in place during the construction process tostabilize the wellbore. The casing forms a major structural component ofthe wellbore and may protect fresh water formations, isolate a zone oflost returns, or isolate formations with significantly differentpressure gradients. Casing may also prevent the formation wall fromcaving into the wellbore, isolate the different formations to preventthe flow or crossflow of formation fluid, and provide a means ofmaintaining control of formation fluids and pressure as the well isdrilled. Casing is usually manufactured from plain carbon steel that isheat-treated to varying strengths, but may be specially fabricated ofstainless steel, aluminum, titanium, fiberglass and other materials.

As used throughout this disclosure, the term “formation” refers to abody of rock that is sufficiently distinctive and continuous from thesurrounding rock bodies that the body of rock can be mapped as adistinct entity. A formation is, therefore, sufficiently homogenous toform a single identifiable unit containing similar rheologicalproperties throughout the formation, including, but not limited to,porosity and permeability. A formation is the fundamental unit oflithostratigraphy.

As used throughout this disclosure, the term “gelation time” refers thetime it takes for a solution to form a gel. Gelation time is a parameterfor screening gel formulations, characterizing gel formulationperformance, and designing gel treatments.

As used throughout this disclosure, the term “hydrolyzable” refers to achemical's ability to be hydrolyzed, to cleave chemical bonds in thepresence of water. Hydrolysis is the reaction of cation, anion, or both,with water molecules, in which the pH of the mixture is altered and thecleavage of H—O bonds takes place.

As used throughout this disclosure, the term “moiety” refers to a partof a molecule.

As used throughout this disclosure, the term “producing formation”refers to the formation from which hydrocarbons are produced.

As used throughout this disclosure, the term “reservoir” refers to asubsurface formation having sufficient porosity and permeability tostore and transmit fluids.

As used throughout this disclosure, the term “target formation” refersto the water-producing formation to be sealed.

As used throughout this disclosure, the term “water coning” refers tothe change in the oil-water contact profile as a result of drawdownpressures during production. Coning may occur in vertical or slightlydeviated wells and is dependent on the characteristics of the fluidsinvolved and the ratio of horizontal to vertical permeability.

As used throughout this disclosure, the term “water cresting” refers tothe change in oil-water contact profile as a result of drawdownpressures during production. Cresting may occur in horizontal or highlydeviated wells and is dependent on the characteristics of the fluidsinvolved and the ratio of horizontal to vertical permeability.

As used throughout this disclosure, the term “water cut” refers to theratio of water produced compared to the volume of total liquidsproduced.

As used throughout this disclosure, the term “wellbore” refers to thedrilled hole or borehole, including the openhole or uncased portion ofthe well. Borehole may refer to the inside diameter of the wellborewall, the rock face that bounds the drilled hole.

To produce hydrocarbons from a hydrocarbon-containing formation,production wells are drilled to a depth that enables these hydrocarbonsto travel from the subsurface formation to the surface. However, someformations in contact with the wellbore may include water, which maylead to water production from the well. High water production, or watercut, is generally undesirable, and sealing the water-producing formationwill decrease the water cut and increase the amount of hydrocarbonsproduced over the lifetime of the well.

The present disclosure is directed to methods and compositions forsealing a subsurface formation. The method includes introducing amixture with a first pH into the subsurface formation. The mixturecomprises an aqueous solution, an alkaline nanosilica dispersion and awater-soluble hydrolyzable compound. The method further includesallowing the water-soluble hydrolyzable compound to hydrolyze in thesubsurface formation to form an acid at 70° C. or greater, therebyacidizing the mixture to a reduced second pH and causing the alkalinenanosilica dispersion to gel into a solid and seal the subsurfaceformation. The composition includes an aqueous mixture including water,an alkaline nanosilica dispersion, and a water-soluble hydrolyzablecompound. The alkaline nanosilica dispersion may include from 5 to 60weight percent (wt. %), from 10 to 60 wt. %, from 20 to 60 wt. %, from30 to 60 wt. %, from 40 to 60 wt. %, from 50 to 60 wt. %, from 5 to 50wt. %, from 10 to 50 wt. %, from 20 to 50 wt. %, from 30 to 50 wt. %,from 40 to 50 wt. %, from 5 to 40 wt. %, from 10 to 40 wt. %, from 20 to40 wt. %, from 30 to 40 wt. %, from 25 to 35 wt. %, from 5 to 30 wt. %,from 10 to 30 wt. %, from 20 to 30 wt. %, from 5 to 20 wt. %, from 10 to20 wt. %, or from 5 to 10 wt. % nanosilica. The weight ratio of thealkaline nanosilica dispersion to the water-soluble hydrolyzablecompound in the aqueous mixture may range from 50:1 to 80:1, from 55:1to 80:1, from 60:1 to 80:1, from 65:1 to 80:1, from 70:1 to 80:1, from75:1 to 80:1, 50:1 to 75:1, from 55:1 to 75:1, from 60:1 to 75:1, from65:1 to 75:1, from 70:1 to 75:1, from 50:1 to 70:1, from 55:1 to 70:1,from 60:1 to 70:1, from 65:1 to 70:1, from 50:1 to 65:1, from 55:1 to65:1, from 60:1 to 65:1, from 50:1 to 60:1, from 55:1 to 60:1, or from50:1 to 55:1. The aqueous mixture may include from 0.1 to 10 volumepercent (vol. %), from 0.5 to 10 vol. %, from 1 to 10 vol. %, from 2 to10 vol. %, from 5 to 10 vol. %, from 8 to 10 vol. %, 0.1 to 8 vol. %,from 0.5 to 8 vol. %, from 1 to 8 vol. %, from 2 to 8 vol. %, from 5 to8 vol. %, from 0.1 to 5 vol. %, from 0.5 to 5 vol. %, from 1 to 5 vol.%, from 2 to 5 vol. %, from 0.1 to 2 vol. %, from 0.5 to 2 vol. %, from1 to 2 vol. %, from 0.1 to 1 vol. %, from 0.5 to 1 vol. %, or from 0.1to 0.5 vol. % of the water-soluble hydrolyzable compound.

Referring now to FIG. 1, an example installation for sealing asubsurface formation is illustrated. As shown in FIG. 1, theinstallation may include a well 100 in contact with a subsurfaceformation 190. The seal 160 formed by the gelled solid 170 of themixture 130, according to the methods of the present disclosure, mayseal at least a portion of the subsurface formation 190 from the well100. Although the seal 160 is depicted in FIG. 1 as impeding horizontalflow from the formation to the well 100, the method and compositionsdescribed in this disclosure may also be used to impede multidirectionalflow, such as vertical flow or combinations of vertical and horizontalflow, for example. Furthermore, although the well 100 is depicted inFIG. 1 as a vertical well, the well 100 may be a horizontal well or adeviated well.

Among other benefits, the mixture may be acceptable for use in regionswith strict environmental regulations. The alkaline nanosilicadispersion component of the mixture is considered environmentally benignand possesses low viscosity, for example, less than 5 centiPoise (cP),during the injection stage. Furthermore, the gelation time of themixture may vary by adjusting the concentration of the hydrolyzablecompound in the mixture. This allows for a predictable gelation timethat may range from a few minutes to several hours at a giventemperature, ranging from 70 to 250° C. Therefore, the injection timemay be controlled based on a variety of circumstances, such as, but notlimited to, how deep into the formation the seal is desired, theformation pressure, the formation temperature, the formation porosity,the formation permeability, the formation anisotropy, the water contentof the formation, and the fracture gradient of the formation. Apredictable gelation time enables the sealant to be precisely placed into the target subsurface formation before the nanosilica gels, therebyavoiding premature plugging of pipelines, coiled tubing or other tubing.

As previously discussed in this disclosure, the mixture includes analkaline nanosilica dispersion. An alkaline nanosilica dispersion is astable dispersion of particles where the particle density and liquidviscosity are such that the particles do not settle. The particles arelarge enough that they would not pass through a membrane or allow othermolecules or ions to pass freely through the dispersion. In oneembodiment, the alkaline nanosilica dispersion is a stable aqueousdispersion. The particles may be silicon dioxide (SiO₂) with particlesizes ranging from 1 to 100 nanometers (nm), from 20 to 80 nm, from 40to 60 nm, from 10 to 50 nm, from 20 to 45 nm, from 1 to 45 nm, from 1 to25 nm, from 5 to 50 nm, from 5 to 15 nm, from 45 to 100 nm, from 45 to85 nm, from 45 to 60 nm, from 45 to 55 nm, from 45 to 50 nm, from 50 to100 nm, from 60 to 90 nm, or from 70 to 80 nm. The alkaline nanosilicadispersion may include from 30 to 50 wt. %, from 40 to 60 wt. %, from 30to 60 wt. %, from 20 to 50 wt. %, from 20 to 60 wt. %, from 40 to 50 wt.%, from 40 to 45 wt. %, from 30 to 40 wt. %, from 35 to 45 wt. %, orfrom 45 to 50 wt. % SiO₂. The alkaline nanosilica dispersion may includefrom 0 to 3 wt. %, from 0.1 to 3 wt. %, from 0.1 to 2.5 wt. %, from 0.1to 2 wt. %, from 0.1 to 1.5 wt. %, from 0.1 to 1 wt. %, from 0.1 to 0.5wt. %, from 0.5 to 3 wt. %, from 0.5 to 2 wt. %, from 0.5 to 1.5 wt. %,from 0.5 to 1 wt. %, from 1 to 1.5 wt. %, from 1 to 2 wt. %, from 1 to2.5 wt. %, from 1 to 3 wt. %, from 1 to 3.5 wt. %, from 1 to 4 wt. %, orfrom 2 to 3 wt. % glycerin. The alkaline nanosilica dispersion mayinclude from 47 to 70 wt. %, from 40 to 50 wt. %, from 40 to 60 wt. %,from 40 to 70 wt. %, from 40 to 80 wt. %, from 47 to 50 wt. %, from 47to 60 wt. %, from 47 to 70 wt. %, from 47 to 80 wt. %, from 50 to 55 wt.%, from 50 to 60 wt. %, from 50 to 70 wt. %, from 50 to 80 wt. %, from60 to 65 wt. %, from 60 to 70 wt. %, from 60 to 75 wt. %, from 60 to 80wt. %, or from 70 to 80 wt. % water. In a specific embodiment, thealkaline nanosilica dispersion may include 45 wt. % SiO₂. The alkalinenanosilica dispersion may appear white or off-white and include aparticle size of 45 nm, a pH at 25° C. of 11, of greater than 7, ofgreater than 9, or of greater than 10, and a specific gravity of 1.32,of from 1.1 to 1.4, of from 1.15 to 1.35, of from 1.2 to 1.35, of from1.25 to 1.35, or of from 1.3 to 1.35.

As previously discussed in this disclosure, the mixture also includes awater-soluble hydrolyzable compound. The mixture may include from 0.25to 4 vol. %, from 0.25 to 0.5 vol. %, from 0.25 to 1 vol. %, from 0.25to 1.5 vol. %, from 0.25 to 2 vol. %, from 0.25 to 2.5 vol. %, from 0.25to 3 vol. %, from 0.25 to 3.5 vol. %, from 0.25 to 4 vol. %, from 0.25to 4.5 vol. %, from 0.25 to 5 vol. %, from 0.5 to 1 vol. %, from 0.5 to1.5 vol. %, from 0.5 to 2 vol. %, from 0.5 to 2.5 vol. %, from 0.5 to 3vol. %, from 0.5 to 3.5 vol. %, from 0.5 to 4 vol. %, from 0.5 to 4.5vol. %, from 0.5 to 5 vol. %, from 1 to 1.5 vol. %, from 1 to 2 vol. %,from 1 to 2.5 vol. %, from 1 to 3 vol. %, from 1 to 3.5 vol. %, from 1to 4 vol. %, from 1 to 4.5 vol. %, from 1 to 5 vol. %, from 1.5 to 2vol. %, from 1.5 to 2.5 vol. %, from 1.5 to 3 vol. %, from 1.5 to 3.5vol. %, from 1.5 to 4 vol. %, from 1.5 to 4.5 vol. %, from 1.5 to 5 vol.%, from 2 to 2.5 vol. %, from 2 to 3 vol. %, from 2 to 3.5 vol. %, from2 to 4 vol. %, from 2 to 4.5 vol. %, from 2 to 5 vol. %, from 3 to 3.5vol. %, from 3 to 4 vol. %, from 3 to 4.5 vol. %, from 3 to 5 vol. %,from 3.5 to 4 vol. %, from 3.5 to 4.5 vol. %, from 3.5 to 5 vol. %, from4 to 4.5 vol. %, or from 4 to 5 vol. % water-soluble hydrolyzablecompound. Similarly, the mixture may include from 600 to 1000 pounds perthousand gallons of fluid (ppt), from 600 to 900 ppt, from 600 to 800ppt, from 600 to 700 ppt, from 700 to 1000 ppt, from 700 to 900 ppt,from 700 to 800 ppt, from 800 to 1000 ppt, from 800 to 900 ppt, from 900to 1000 ppt, or approximately 759.45 ppt water-soluble hydrolyzablecompound. Various water-soluble hydrolyzable compounds are contemplatedand may include an organic salt, for example. Organic salts are a saltcontaining an organic ion, that is, an ion that contains a carbon atom.The organic salt may include a carboxylic acid moiety, an organiccompound that includes a carboxyl group. When hydrolyzed, thewater-soluble hydrolyzable compound including the carboxylic acid moietyforms a carboxylic acid. Various organic salts are contemplated and mayinclude diethylene glycol diformate, ethyl acetate, ethyl formate,ethylene glycol diacetate, or diethylene glycol dilactate. Whenhydrolyzed, diethylene glycol diformate and ethyl formate generateformic acid. When hydrolyzed, ethyl acetate and ethylene glycoldiacetate generate acetic acid. When hydrolyzed, diethylene glycoldilactate forms lactic acid.

As previously discussed in this disclosure, the mixture further includesan aqueous solution. In some embodiments, the aqueous solution mayinclude one or more than one of fresh water, salt water, brine, connatebrine, municipal water, formation water, produced water, well water,filtered water, distilled water, and sea water. In some embodiments, theaqueous solution may include water or a solution containing water andone or more inorganic compounds dissolved in the water or otherwisecompletely miscible with the water. In some embodiments, the aqueoussolution may contain brine, including natural and synthetic brine. Brineincludes water and a salt that may include calcium chloride, calciumbromide, sodium chloride, sodium bromide, other salts, and combinationsof these. The aqueous solution may include total dissolved solids offrom 150,000 to 300,000 milligrams per liter (mg/L) (150 to 300kiligrams per cubic meter (kg/m³); 1,000 mg/L=1 kg/m³).

The mixture may further include a filtration control additive, aviscosifier, a pH control additive, or combinations of these. Thefiltration control additive may include at least one of starch, such asDextrid®, produced by Halliburton, and polyanionic cellulose, such asPAC R, produced by Schlumberger. The starch may include a modified andbacterially stabilized starch product used to reduce mud filtrate inmost water-based mud systems. The starch may be functional in freshwaterand saturated salt environments, does not increase fluid viscosity, istemperature stable, meaning that it will not chemically degrade, toapproximately 121° C., and may be used in both drilling fluid andreservoir drill-in fluid applications. The starch may be used toencapsulate drill cuttings and exposed wellbore formations to reduceparticle dispersion and reactive clay/shale formation swelling. Thestarch may have a specific gravity of 1.5. The addition of a filtrationcontrol additive results in several benefits, such as reduced filtrationrates, improved borehole stability, filtration control withoutdetrimental viscosity increase, and decreased clay dispersion, asnonlimiting examples.

The viscosifier may include at least one of bentonite and xanthan gum,also known as XC polymer. Bentonite is a material composed of clayminerals, predominantly montmorillonite with minor amounts of othersmectite group minerals, conventionally used in drilling mud. Bentoniteswells considerably when exposed to water, making it ideal forprotecting formations from invasion by drilling fluids. Xanthan gum is ahigh-molecular weight biopolymer (ranging from approximately 1,500,000to 2,500,000 grams per mole), and provides versatile rheology control ina wide range of brines and drilling fluids. Xanthan gum comprises a highviscosity at low shear, which may result in continuous suspension ofsolids in low concentration drilling fluids even at low shear. The heatresistance present in xanthan gum makes it a reliable displacing agentand mobile control agent. Xanthan gum is considered non-hazardous andsuitable for use in environmentally sensitive locations andapplications. Increasing the viscosity of the mixture through theaddition of viscosifiers results in several benefits, such as minimizedpumping friction in lime, freshwater and saltwater muds, decreaseddamage to oil formation, decreased maintenance expense, decreased totalcost of operation, and stabilized uniform suspension of particles, asnonlimiting examples.

The pH control additive may include sodium hydroxide, or caustic soda,which, as a strong alkaline compound, controls the alkalinity insodium-based clear brine fluids. Sodium hydroxide may be used to helpsolubilize acidic additive compounds that are difficult to dissolve inneutral to low pH environments. Without being limited by theory,increasing the pH of the mixture through the addition of a pH controladditive may affect the gelation time of the mixture, as the alkalinenanosilica dispersion gels when the mixture reaches an acidic, secondpH. As previously discussed in this disclosure, a predictable gelationtime enables the sealant to be precisely placed in to the targetsubsurface formation before the nanosilica gels, and directly affectsthe injection time. Therefore, the gelation time of the mixture may bedetermined based on the amount of pH control additive in the mixture andby the amount of water-soluble hydrolyzable compound in the mixture.

The mixture has an initial viscosity that enables the mixture to beinjected a farther distance into the formation compared to conventionalsealing materials. In some embodiments, the aqueous mixture may have aviscosity of less than 5 cp (5 milliPascal seconds (mPa·s); 1 cp=1mPa·s). For example, in some embodiments, the mixture may have aviscosity of from 0.1 cP to 100 cP, 0.1 cP to 70 cP, from 0.1 cP to 40cP, from 0.1 cP to 20 cP, from 0.1 cP to 10 cP, from 0.1 cP to 5 cP,from 0.1 cP to 3 cP, from 0.1 cP to 2 cP, from 0.1 cP to 1 cP, from 0.1cP to 0.5 cP, from 0.5 cP to 5 cP, from 0.5 cP to 3 cP, from 0.5 cP to 2cP, from 0.5 cP to 1 cP, from 1 cP to 5 cP, or from 1 cP to 3 cP.

In one embodiment, the composition of the present disclosure includes ahomogenous aqueous mixture including water, an alkaline nanosilicadispersion, and a water-soluble hydrolyzable compound. The aqueousmixture may be homogenous.

Referring again to FIG. 1, as previously discussed in this disclosure,the method includes introducing the mixture 130 into the subsurfaceformation 190. The mixture 130 includes a first pH of greater than 7, orof greater than 9, or of 9 to 11, or of approximately 10. Introducingthe mixture 130 into the subsurface formation 190 may include injectingthe mixture 130 into the subsurface formation 190. The mixture 130 maybe injected into the subsurface formation 190 by a well 100. After beingintroduced into the subsurface formation, the hydrolyzable compoundwithin the mixture 130 hydrolyzes within the subsurface formation 190 toform an acid at 70° C. or greater, thereby acidizing the mixture 130 toa second, acidic, pH and causing the alkaline nanosilica dispersion togel into a gelled solid 170 and seal the subsurface formation 190. Thesecond pH of the mixture 130 may be less than 7, less than 5, less than4, or approximately 3. The subsurface formation 190 may have atemperature of from 70° C. to 250° C., and this temperature may activatethe water-soluble hydrolyzable compound. The subsurface formation mayinclude carbonate or sandstone. The gelation time for the mixture 130 toform a gelled solid 170 to create the seal 160 in the subsurfaceformation 190 ranges from 2 minutes to 5 hours. The method may be usedto seal the wellbore from downhole water coning or water cresting, toseal the channel behind the casing, to seal the channel from theinjector, to seal the wellbore from cross flow, or to seal naturalfractures. The method may further result in less than 100 gallons, lessthan 10 gallons, less than 5 gallons, less than 3 gallons, or less than1 gallon of fluid breakthrough.

EXAMPLES

The following example illustrates features of the present disclosure butis not intended to limit the scope of the disclosure.

Example 1

To test the capacity of the alkaline nanosilica dispersion to form agelled solid when combined with a water-soluble hydrolyzable compoundthat forms an acid at greater than 70° C. or greater, 100 mL of alkalinenanosilica dispersion and 2 gm of diethylene glycol diformate were mixedwell using a stirrer. The alkaline nanosilica dispersion along with thediethylene glycol diformate was subjected to static aging at 200° F.(93° C.; T_((° C.))=(T_((° F.))−32)×5/9) for 16 hours. After 16 hours ofstatic aging, the alkaline nanosilica dispersion was converted into asolid.

Example 2

In order to examine the ability of the nanosilica gelled solid to plugand seal a water producing zone, filtration tests were conducted onthree mixtures using an Ofite HTHP Filter Press. The compositions of thethree mixtures are given in Tables 1A-1C. The components are measured bygallons per thousand gallons of fluid (gpt) or pounds per thousandgallons of fluid (ppt).

TABLE 1A Comparative mixture 1 contains only water, caustic soda, andalkaline nanosilica dispersion, and does not contain a water-solublehydrolyzable compound or conventional filtration control additives.Mixing time Additives (min) Amount Function Water   408 gpt CarrierCaustic soda 5  0.71 ppt pH Control Alkaline 5 759.45 ppt Sealantnanosilica dispersion

TABLE 1B Comparative mixture 2 contains water, caustic soda,conventional filtration control additives, and alkaline nanosilicadispersion, but does not contain a water- soluble hydrolyzable compound.Mixing time Additives (min) Amount Function Water  408 gpt CarrierBentonite 20 11.42 ppt  viscosifier XC polymer 5 5.71 ppt Biopolymerviscosifer Starch 5 11.42 ppt  Filtration control additive Pac R 5 1.42gpt Filtration control additive Caustic soda 5 0.71 ppt pH ControlAlkaline 5 759.45 ppt  Sealant nanosilica dispersion

TABLE 1C Inventive mixture 3 contains water, caustic soda, conventionalfiltration control additives, alkaline nanosilica dispersion, and awater-soluble hydrolyzable compound, diethylene glycol diformate. Mixingtime Additives (min) Amount Function Water  408 gpt Carrier Bentonite 2011.42 ppt  viscosifier XC polymer 5 5.71 ppt Biopolymer viscosiferStarch 5 11.42 ppt  Filtration control additive Pac R 5 1.42 pptFiltration control additive Caustic soda 5 0.71 ppt pH Control Alkalinenanosilica 5 759.45 ppt  Sealant dispersion with diethylene glycoldiformate hydrolyzable compound

After mixing, the respective viscosities of comparative mixture 1,comparative mixture 2, and inventive mixture 3 were measured at 511inverse seconds (s⁻¹) at 120° F. with a Fann 35 rheometer. Theseviscosities are shown below in Table 2.

TABLE 2 Viscosities for comparative mixture 1, comparative mixture 2,and inventive mixture 3. Fluid Viscosity (cp) Comparative mixture 1 5Comparative mixture 2 43 Inventive mixture 3 44

Next, 250 milliLiters (mL) of each mixture was transferred to a filterpress to evaluate ability of each mixture to plug and seal. The OfiteHTHP Filter Press was pressurized to 600 pounds per square inch (psi)with a back pressure of 100 psi, thereby maintaining a pressuredifferential of 500 psi and heated to 200° F. The mixture was shut infor 24 hours. After 24 hours, fluid loss was measured for 30 minutes.Table 3 provides results of the fluid loss for comparative mixture 1,comparative mixture 2, and inventive mixture 3.

TABLE 3 Fluid loss values for comparative mixture 1, comparative mixture2, and inventive mixture 3. Fluid loss Fluid (mL) Comparative mixture 1245 Comparative mixture 2 5 Inventive mixture 3 0

Furthermore, addition of conventional filtration control additives andthe conformance sealant's compatibility with them would mitigate anyinitial fluid loss before the sealant sets in to a gel.

It was observed that comparative mixture 1 had the worst fluid loss.Comparative mixture 2 showed a controlled fluid loss due the presence ofconventional filtration control additives. Finally, inventive mixture 3,which contained conventional filtration control additives, the alkalinenanosilica dispersion, and the water-soluble hydrolyzable compound,completely prevented any fluid loss by plugging filter disk. To furtherconfirm the sealant's plugging ability, the Ofite HTHP Filter Press wascooled and depressurized. Water was injected from the top under apressure of 20 psi. No water breakthrough was observed under thispressure.

For the purposes of describing and defining the present invention, it isnoted that reference in this application to a characteristic of thesubject matter of the present disclosure being a “function of” aparameter, variable, or other characteristic is not intended to denotethat the characteristic is exclusively a function of the listedparameter, variable, or characteristic. Rather, reference in thisapplication to a characteristic that is a “function” of a listedparameter, variable, etcetera, is intended to be open ended such thatthe characteristic may be a function of a single parameter, variable,etc., or a plurality of parameters, variables, etcetera.

It is also noted that recitations in this application of “at least one”component, element, etcetera, should not be used to create an inferencethat the alternative use of the articles “a” or “an” should be limitedto a single component, element, etcetera.

For the purposes of describing and defining the present invention it isnoted that the terms “substantially” and “approximately” are utilized inthis application to represent the inherent degree of uncertainty thatmay be attributed to any quantitative comparison, value, measurement, orother representation. The terms “substantially” and “approximately” arealso utilized in this application to represent the degree by which aquantitative representation may vary from a stated reference withoutresulting in a change in the basic function of the subject matter atissue.

Having described the subject matter of the present disclosure in detailand by reference to specific embodiments, it is noted that the variousdetails disclosed in this application should not be taken to imply thatthese details relate to elements that are essential components of thevarious embodiments described in this application, even in cases where aparticular element is illustrated in each of the drawings that accompanythe present description. Further, it will be apparent that modificationsand variations are possible without departing from the scope of thepresent disclosure, including, but not limited to, embodiments definedin the appended claims. More specifically, although some aspects of thepresent disclosure are identified in this application as preferred orparticularly advantageous, it is contemplated that the presentdisclosure is not necessarily limited to these aspects.

What is claimed is:
 1. A method of sealing a subsurface formationcomprising: introducing a mixture with a first pH into the subsurfaceformation in which the mixture comprises: an aqueous solution, afiltration control additive comprising at least one of starch andpolyanionic cellulose, sodium hydroxide, an alkaline nanosilicadispersion, and a water-soluble hydrolyzable organic salt selected fromdiethylene glycol diformate, ethylene glycol diacetate, and diethyleneglycol dilactate; and allowing the water-soluble hydrolyzable organicsalt to hydrolyze in the subsurface formation to form an acid at 70° C.or greater, thereby acidizing the mixture to a reduced second pH andcausing the alkaline nanosilica dispersion to gel into a solid and sealthe subsurface formation.
 2. The method of claim 1, in which the mixturefurther comprises a viscosifier comprising at least one of bentonite andxanthan gum.
 3. The method of claim 1, in which the aqueous solutioncomprises at least one of fresh water, salt water, brine, municipalwater, formation water, produced water, well water, filtered water,distilled water, sea water, or combinations of these.
 4. The method ofclaim 1, in which the aqueous solution comprises total dissolved solidsof from 150,000 to 300,000 mg/L.
 5. The method of claim 1, in whichcausing the alkaline nanosilica dispersion to gel into a solid and sealthe subsurface formation thereby reduces an amount of fluid breakthroughfrom the subsurface formation.
 6. The method of claim 1, in which thesubsurface formation has a temperature of from 70 to 250° C.
 7. Themethod of claim 6, in which the temperature of the subsurface formationactivates the water-soluble hydrolyzable organic salt.
 8. The method ofclaim 1, in which the first pH is greater than
 7. 9. The method of claim1, in which the second pH is less than
 7. 10. The method of claim 1, inwhich the water-soluble hydrolyzable organic salt is diethylene glycoldiformate and the acid is formic acid.
 11. The method of claim 1, inwhich a gelation time of the mixture in the subsurface formation rangesfrom 2 minutes to 5 hours.
 12. The method of claim 1, in which thesubsurface formation comprises carbonate or sandstone.
 13. The method ofclaim 1, in which a viscosity of the mixture is 5 cP or less whenintroducing the mixture into the subsurface formation.
 14. The method ofclaim 1, in which the alkaline nanosilica dispersion is from 30 to 60wt. % silicon dioxide.
 15. The method of claim 1, in which the weightratio of the alkaline nanosilica dispersion to the water-solublehydrolyzable organic salt is from 50:1 to 80:1.
 16. The method of claim1, in which the water-soluble hydrolyzable organic salt is selected fromdiethylene glycol diformate and diethylene glycol dilactate.